In the ultrasound measurement of blood flow, a hand held or headpiece mounted probe is typically used to transmit a pulsed beam of ultrasound through body tissue to a focal point within a target artery. Blood cells flowing through the artery, being denser than surrounding tissue, scatter the ultrasound in many directions. A portion of the transmitted ultrasound is reflected directly back to the probe, which can then function as a receiver.
In accordance with the well known Doppler phenomenon, an observer in motion relative to a wave source will receive a wave from the source which has a frequency different than the frequency of the wave at the source. If the source is moving toward the observer, a higher frequency wave is received by the observer, and, conversely, if the wave source is moving away from the observer, a lower frequency wave is received. The difference between the emitted and received frequencies is known as the Doppler shift.
Instruments have been developed to obtain noninvasive measurements of blood velocity in interior arteries and veins using Doppler principles. One instrument in present use for obtaining transcranial Doppler measurements is the TC 2000S Transcranial Doppler System available from the EME (Eden Medizinische Electronik GmbH) division of Nicolet Instrument Corporation. The operation of such instruments in accordance with the Doppler principle may be illustrated with respect to FIGS. 1-4. In FIG. 1, the ultrasound probe 40 acts as a stationary wave source, emitting pulsed ultrasound at a frequency of, e.g., 2 MHz. This ultrasound is transmitted through the skull 41 and the tissue of the brain to a blood vessel 42. For purposes of illustration, a blood cell 43 is shown moving toward the probe and acts as a moving observer. As illustrated in FIG. 2, the blood cell reflects the pulse of ultrasound and can be considered a moving wave source. The probe receives this reflected ultrasound, acting as a stationary observer. The frequency of the ultrasound received by the probe, f.sub.1, is higher than the frequency, f.sub.0, originally emitted. The Doppler shift of the received wave can then be calculated. FIG. 3 shows the effect on a pulse of ultrasound when blood flows in a direction away from the probe. In this case, the received frequency, f.sub.2, reflected from the blood cell, is lower than the emitted frequency f.sub.0. Again, the Doppler shift can be calculated.
The Doppler effect can be used to determine the velocity of blood flow in the cerebral arteries. For this purpose, the Doppler equation used is the following: ##EQU1## where: F.sub.d =Doppler frequency shift
F.sub.t =Frequency of the transmitter PA1 V=Velocity of blood flow PA1 .theta.=Angle of incidence between the probe and the artery PA1 V.sub.0 =Velocity of ultrasound in body tissue
Typically, F.sub.t is a constant, e.g., 2, 4 or 8 MHz, and V.sub.0 is approximately 1540 meters per second (m/s) in soft body tissue.
Assuming that there is a zero angle of incidence between the probe and the artery, the value of cos .theta. is equal to 1. The effect of the angle .theta. is only significant for angles of incidence exceeding 30.degree..
In the TC 2000S System, ultrasonic energy is provided in bursts at a pulse repetition rate or frequency. The probe receives the echoes from each burst and converts the sound energy to an electrical signal. To obtain signal data corresponding to reflections occurring at a specific depth (range) within the head, an electronic gate opens to receive the reflected signal at a selected time after the excitation pulse, corresponding to the expected time of arrival of an echo from a position at the selected depth. The range resolution is generally limited by the bandwidth of the various components of the instrument and the length of the burst. The bandwidth can be reduced by filtering the received signal, but at the cost of an increased length of sample volume.
Other body movements, for example, vessel wall contractions, can also scatter ultrasound which will be detected as "noise" in the Doppler signal. To reduce this noise interference, a high pass filter is used to reduce the low frequency, high amplitude signals. The high pass filter typically can be adjusted to have a passband above a cutoff frequency selectable between 0 and, e.g., 488 Hz.
Because not all blood cells in the sample volume are moving at the same speed, a range or spectrum of Doppler shifted frequencies are reflected back to the probe. Thus, the signal from the probe 40 may be converted to digital form by an analog-to-digital converter, and the spectral content of the sampled Doppler signal calculated by a computer or digital signal processor using a fast Fourier transform method. This processing method produces a velocity profile of the blood flow, which varies over the period of a heartbeat. The process is repeated to produce a beat-to-beat flow pattern, or sonogram, on a video display. The TC 2000S instrument can be configured to analyze 64 or 128 separate frequency ranges within the spectrum of Doppler signals. Color coding may be used to show the intensity of the signal at different points on the spectral line. The intensity of the signal will represent the proportion of blood cells flowing within that particular velocity range. The information displayed on the video screen can be used by a trained observer to determine blood flow characteristics at particular positions within the brain of the individual being tested, and can detect anomalies in such blood flow, for example, the possible presence of a blockage or restriction, or the passage of an embolus through the artery which introduces a transient distortion of the displayed information. The TC 2000S also includes a processing option which provides a maximum frequency follower or envelope curve which is displayed on the video screen as the white outline of the flow spectrum.
The usefulness of the information obtained with such transcranial Doppler instruments can be affected if significant noise is present. It is therefore desirable to improve the signal-to-noise ratio (SNR) of the received data in a manner which does not significantly compromise the desired signal data which is representative of blood flow velocity. A better spectral display can sometimes be obtained by performing many fast Fourier transforms (FFT) on the data and averaging these to reduce the random fluctuations of the spectral amplitudes or by performing a sliding FFT on overlapping data sets and averaging several of these FFTs. The background noise is reduced somewhat in this manner, which facilitates tracing of the spectral outline, but significant noise is often present which it is desirable to further reduce.